I also really like the chatbox, especially with its archive function. I can save large discussions that way (mwahahaha! XD). The one thing I do really like about the Skype app on Windows 10 is the notifications it gives me. It shows me a pop-up of the message right on my screen no matter what tab I'm on, and oftentimes I don't even bother to pull up my Skype app to reply since I can do it through the pop-up. In the chatbox, when I'm in a different tab, I don't hear the sound of a new reply every time and then I don't notice when someone's talking to me, which kinda sucks. Also, it's nice having different Skype groups for the rps, especially the Avatar rp. I know that we kind of annoyed some people when we were constantly discussing it in the chatbox. Then again, new members will often be in the chatbox, but they usually don't join the Skype group, at least not right away. So if we're talking in the chatbox, then they have a chance to jump in with us (although they usually don't haha). Overall, for convenience and ease of seeing/replying to messages, Skype is better, but I also want to see the chatbox restored to its former glory, so I'm kinda torn, sorry.
As for Discord vs. Skype, I don't know much about Discord and have never used it, but I will say that the Skype app on Windows 10 is pretty good. Except you can't change your freaking display name lol.